In many areas of modern society, a reliable power supply is crucial for the correct functioning of electrical appliances. One such area is mobile radio telephony, where the users of mobile radio telephones expect the mobile radio network to function without disturbances. In such mobile radio networks, switched mode power supplies are used in order to convert the mains voltage into a steady DC voltage which can be provided to various loads, such as e.g. radio transceivers. In a switched mode power supply, transistors are switched on and off according to a variable duty cycle, in order to obtain the desired output characteristics of the power supply. Furthermore, a switched mode power supply is often further connected to a battery, in order to charge the battery so that the battery can take over the power supply should the mains voltage fail. An example of a switched mode power supply is described in A new improved rectifier for standard telecom systems, Telescon 97, Budapest Apr. 22-24, 1997.